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                <h1 class="post-title" itemprop="name headline"> [Python3网络爬虫开发实战] 13.5–Downloader Middleware 的用法 </h1>
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                <h1 id="13-5-Downloader-Middleware-的用法"><a href="#13-5-Downloader-Middleware-的用法" class="headerlink" title="13.5 Downloader Middleware 的用法"></a>13.5 Downloader Middleware 的用法</h1>
                <p>Downloader Middleware 即下载中间件，它是处于 Scrapy 的 Request 和 Response 之间的处理模块。我们首先来看看它的架构，如图 13-1 所示。 Scheduler 从队列中拿出一个 Request 发送给 Downloader 执行下载，这个过程会经过 Downloader Middleware 的处理。另外，当 Downloader 将 Request 下载完成得到 Response 返回给 Spider 时会再次经过 Downloader Middleware 处理。 也就是说，Downloader Middleware 在整个架构中起作用的位置是以下两个。</p>
                <ul>
                  <li>在 Scheduler 调度出队列的 Request 发送给 Downloader 下载之前，也就是我们可以在 Request 执行下载之前对其进行修改。</li>
                  <li>在下载后生成的 Response 发送给 Spider 之前，也就是我们可以在生成 Resposne 被 Spider 解析之前对其进行修改。</li>
                </ul>
                <p>Downloader Middleware 的功能十分强大，修改 User-Agent、处理重定向、设置代理、失败重试、设置 Cookies 等功能都需要借助它来实现。下面我们来了解一下 Downloader Middleware 的详细用法。</p>
                <h3 id="1-使用说明"><a href="#1-使用说明" class="headerlink" title="1. 使用说明"></a>1. 使用说明</h3>
                <p>需要说明的是，Scrapy 其实已经提供了许多 Downloader Middleware，比如负责失败重试、自动重定向等功能的 Middleware，它们被 DOWNLOADER_MIDDLEWARES_BASE 变量所定义。 DOWNLOADER_MIDDLEWARES_BASE 变量的内容如下所示：</p>
                <figure class="highlight yaml">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line"><span class="string">&#123;</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.robotstxt.RobotsTxtMiddleware':</span> <span class="number">100</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.httpauth.HttpAuthMiddleware':</span> <span class="number">300</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.downloadtimeout.DownloadTimeoutMiddleware':</span> <span class="number">350</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.defaultheaders.DefaultHeadersMiddleware':</span> <span class="number">400</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.useragent.UserAgentMiddleware':</span> <span class="number">500</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.retry.RetryMiddleware':</span> <span class="number">550</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.ajaxcrawl.AjaxCrawlMiddleware':</span> <span class="number">560</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.redirect.MetaRefreshMiddleware':</span> <span class="number">580</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.httpcompression.HttpCompressionMiddleware':</span> <span class="number">590</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.redirect.RedirectMiddleware':</span> <span class="number">600</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.cookies.CookiesMiddleware':</span> <span class="number">700</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.httpproxy.HttpProxyMiddleware':</span> <span class="number">750</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.stats.DownloaderStats':</span> <span class="number">850</span><span class="string">,</span></span><br><span class="line">    <span class="attr">'scrapy.downloadermiddlewares.httpcache.HttpCacheMiddleware':</span> <span class="number">900</span><span class="string">,</span></span><br><span class="line"><span class="string">&#125;</span></span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>这是一个字典格式，字典的键名是 Scrapy 内置的 Downloader Middleware 的名称，键值代表了调用的优先级，优先级是一个数字，数字越小代表越靠近 Scrapy 引擎，数字越大代表越靠近 Downloader。每个 Downloader Middleware 都可以定义 process_request() 和 request_response() 方法来分别处理请求和响应，对于 process_request() 方法来说，优先级数字越小越先被调用，对于 process_response() 方法来说，优先级数字越大越先被调用。。 如果自己定义的 Downloader Middleware 要添加到项目里，DOWNLOADER_MIDDLEWARES_BASE 变量不能直接修改。Scrapy 提供了另外一个设置变量 DOWNLOADER_MIDDLEWARES，我们直接修改这个变量就可以添加自己定义的 Downloader Middleware，以及禁用 DOWNLOADER_MIDDLEWARES_BASE 里面定义的 Downloader Middleware。下面我们具体来看看 Downloader Middleware 的使用方法。</p>
                <h3 id="2-核心方法"><a href="#2-核心方法" class="headerlink" title="2. 核心方法"></a>2. 核心方法</h3>
                <p>Scrapy 内置的 Downloader Middleware 为 Scrapy 提供了基础的功能，但在项目实战中我们往往需要单独定义 Downloader Middleware。不用担心，这个过程非常简单，我们只需要实现某几个方法即可。 每个 Downloader Middleware 都定义了一个或多个方法的类，核心的方法有如下三个。</p>
                <ul>
                  <li>process_request(request, spider)</li>
                  <li>process_response(request, response, spider)</li>
                  <li>process_exception(request, exception, spider)</li>
                </ul>
                <p>我们只需要实现至少一个方法，就可以定义一个 Downloader Middleware。下面我们来看看这三个方法的详细用法。</p>
                <h4 id="process-request-request-spider"><a href="#process-request-request-spider" class="headerlink" title="process_request(request, spider)"></a>process_request(request, spider)</h4>
                <p>Request 被 Scrapy 引擎调度给 Downloader 之前，process_request() 方法就会被调用，也就是在 Request 从队列里调度出来到 Downloader 下载执行之前，我们都可以用 process_request() 方法对 Request 进行处理。方法的返回值必须为 None、Response 对象、Request 对象之一，或者抛出 IgnoreRequest 异常。 process_request() 方法的参数有如下两个。</p>
                <ul>
                  <li>request，即 Request 对象，即被处理的 Request</li>
                  <li>spider，即 Spdier 对象，即此 Request 对应的 Spider</li>
                </ul>
                <p>返回类型不同，产生的效果也不同。下面归纳一下不同的返回情况。</p>
                <ul>
                  <li>当返回是 None 时，Scrapy 将继续处理该 Request，接着执行其他 Downloader Middleware 的 process_request() 方法，一直到 Downloader 把 Request 执行后得到 Response 才结束。这个过程其实就是修改 Request 的过程，不同的 Downloader Middleware 按照设置的优先级顺序依次对 Request 进行修改，最后送至 Downloader 执行。</li>
                  <li>当返回为 Response 对象时，更低优先级的 Downloader Middleware 的 process_request() 和 process_exception() 方法就不会被继续调用，每个 Downloader Middleware 的 process_response() 方法转而被依次调用。调用完毕之后，直接将 Response 对象发送给 Spider 来处理。</li>
                  <li>当返回为 Request 对象时，更低优先级的 Downloader Middleware 的 process_request() 方法会停止执行。这个 Request 会重新放到调度队列里，其实它就是一个全新的 Request，等待被调度。如果被 Scheduler 调度了，那么所有的 Downloader Middleware 的 process_request() 方法会被重新按照顺序执行。</li>
                  <li>如果 IgnoreRequest 异常抛出，则所有的 Downloader Middleware 的 process_exception() 方法会依次执行。如果没有一个方法处理这个异常，那么 Request 的 errorback() 方法就会回调。如果该异常还没有被处理，那么它便会被忽略。</li>
                </ul>
                <h4 id="process-response-request-response-spider"><a href="#process-response-request-response-spider" class="headerlink" title="process_response(request, response, spider)"></a>process_response(request, response, spider)</h4>
                <p>Downloader 执行 Request 下载之后，会得到对应的 Response。Scrapy 引擎便会将 Response 发送给 Spider 进行解析。在发送之前，我们都可以用 process_response() 方法来对 Response 进行处理。方法的返回值必须为 Request 对象、Response 对象之一，或者抛出 IgnoreRequest 异常。 process_response() 方法的参数有如下三个。</p>
                <ul>
                  <li>request，是 Request 对象，即此 Response 对应的 Request。</li>
                  <li>response，是 Response 对象，即此被处理的 Response。</li>
                  <li>spider，是 Spider 对象，即此 Response 对应的 Spider。</li>
                </ul>
                <p>下面对不同的返回情况做一下归纳：</p>
                <ul>
                  <li>当返回为 Request 对象时，更低优先级的 Downloader Middleware 的 process_response() 方法不会继续调用。该 Request 对象会重新放到调度队列里等待被调度，它相当于一个全新的 Request。然后，该 Request 会被 process_request() 方法顺次处理。</li>
                  <li>当返回为 Response 对象时，更低优先级的 Downloader Middleware 的 process_response() 方法会继续调用，继续对该 Response 对象进行处理。</li>
                  <li>如果 IgnoreRequest 异常抛出，则 Request 的 errorback() 方法会回调。如果该异常还没有被处理，那么它便会被忽略。</li>
                </ul>
                <h4 id="process-exception-request-exception-spider"><a href="#process-exception-request-exception-spider" class="headerlink" title="process_exception(request, exception, spider)"></a>process_exception(request, exception, spider)</h4>
                <p>当 Downloader 或 process_request() 方法抛出异常时，例如抛出 IgnoreRequest 异常，process_exception() 方法就会被调用。方法的返回值必须为 None、Response 对象、Request 对象之一。 process_exception() 方法的参数有如下三个。</p>
                <ul>
                  <li>request，即 Request 对象，即产生异常的 Request</li>
                  <li>exception，即 Exception 对象，即抛出的异常</li>
                  <li>spdier，即 Spider 对象，即 Request 对应的 Spider</li>
                </ul>
                <p>下面归纳一下不同的返回值。</p>
                <ul>
                  <li>当返回为 None 时，更低优先级的 Downloader Middleware 的 process_exception() 会被继续顺次调用，直到所有的方法都被调度完毕。</li>
                  <li>当返回为 Response 对象时，更低优先级的 Downloader Middleware 的 process_exception() 方法不再被继续调用，每个 Downloader Middleware 的 process_response() 方法转而被依次调用。</li>
                  <li>当返回为 Request 对象时，更低优先级的 Downloader Middleware 的 process_exception() 也不再被继续调用，该 Request 对象会重新放到调度队列里面等待被调度，它相当于一个全新的 Request。然后，该 Request 又会被 process_request() 方法顺次处理。</li>
                </ul>
                <p>以上内容便是这三个方法的详细使用逻辑。在使用它们之前，请先对这三个方法的返回值的处理情况有一个清晰的认识。在自定义 Downloader Middleware 的时候，也一定要注意每个方法的返回类型。 下面我们用一个案例实战来加深一下对 Downloader Middleware 用法的理解。</p>
                <h3 id="3-项目实战"><a href="#3-项目实战" class="headerlink" title="3. 项目实战"></a>3. 项目实战</h3>
                <p>新建一个项目，命令如下所示：</p>
                <figure class="highlight ebnf">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line"><span class="attribute">scrapy startproject scrapydownloadertest</span></span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>新建了一个 Scrapy 项目，名为 scrapydownloadertest。进入项目，新建一个 Spider，命令如下所示：</p>
                <figure class="highlight avrasm">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line">scrapy genspider httpbin httpbin<span class="meta">.org</span></span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>新建了一个 Spider，名为 httpbin，源代码如下所示：</p>
                <figure class="highlight ruby">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line">import scrapy</span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">HttpbinSpider</span>(<span class="title">scrapy</span>.<span class="title">Spider</span>):</span></span><br><span class="line">    name = <span class="string">'httpbin'</span></span><br><span class="line">    allowed_domains = [<span class="string">'httpbin.org'</span>]</span><br><span class="line">    start_urls = [<span class="string">'http://httpbin.org/'</span>]</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">def</span> <span class="title">parse</span><span class="params">(<span class="keyword">self</span>, response)</span></span><span class="symbol">:</span></span><br><span class="line">        pass</span><br><span class="line"><span class="string">``</span><span class="string">`接下来我们修改 start_urls 为：`</span>[<span class="string">'http://httpbin.org/'</span>]<span class="string">`。随后将 parse() 方法添加一行日志输出，将 response 变量的 text 属性输出出来，这样我们便可以看到 Scrapy 发送的 Request 信息了。</span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string">修改 Spider 内容如下所示：</span></span><br><span class="line"><span class="string"></span></span><br><span class="line"><span class="string">`</span><span class="string">``</span>python</span><br><span class="line">import scrapy</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">HttpbinSpider</span>(<span class="title">scrapy</span>.<span class="title">Spider</span>):</span></span><br><span class="line">    name = <span class="string">'httpbin'</span></span><br><span class="line">    allowed_domains = [<span class="string">'httpbin.org'</span>]</span><br><span class="line">    start_urls = [<span class="string">'http://httpbin.org/get'</span>]</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">def</span> <span class="title">parse</span><span class="params">(<span class="keyword">self</span>, response)</span></span><span class="symbol">:</span></span><br><span class="line">        <span class="keyword">self</span>.logger.debug(response.text)</span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>接下来运行此 Spider，执行如下命令：</p>
                <figure class="highlight ebnf">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line"><span class="attribute">scrapy crawl httpbin</span></span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>Scrapy 运行结果包含 Scrapy 发送的 Request 信息，内容如下所示：</p>
                <figure class="highlight json">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line">&#123;<span class="attr">"args"</span>: &#123;&#125;, </span><br><span class="line">  <span class="attr">"headers"</span>: &#123;</span><br><span class="line">    <span class="attr">"Accept"</span>: <span class="string">"text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"</span>, </span><br><span class="line">    <span class="attr">"Accept-Encoding"</span>: <span class="string">"gzip,deflate,br"</span>, </span><br><span class="line">    <span class="attr">"Accept-Language"</span>: <span class="string">"en"</span>, </span><br><span class="line">    <span class="attr">"Connection"</span>: <span class="string">"close"</span>, </span><br><span class="line">    <span class="attr">"Host"</span>: <span class="string">"httpbin.org"</span>, </span><br><span class="line">    <span class="attr">"User-Agent"</span>: <span class="string">"Scrapy/1.4.0 (+http://scrapy.org)"</span></span><br><span class="line">  &#125;, </span><br><span class="line">  <span class="attr">"origin"</span>: <span class="string">"60.207.237.85"</span>, </span><br><span class="line">  <span class="attr">"url"</span>: <span class="string">"http://httpbin.org/get"</span></span><br><span class="line">&#125;</span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>我们观察一下 Headers，Scrapy 发送的 Request 使用的 User-Agent 是 Scrapy/1.4.0(+<a href="http://scrapy.org" target="_blank" rel="noopener">http://scrapy.org)，这其实是由</a>，这其实是由) Scrapy 内置的 UserAgentMiddleware 设置的，UserAgentMiddleware 的源码如下所示：</p>
                <figure class="highlight ruby">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line">from scrapy import signals</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">UserAgentMiddleware</span>(<span class="title">object</span>):</span></span><br><span class="line">    <span class="function"><span class="keyword">def</span> <span class="title">__init__</span><span class="params">(<span class="keyword">self</span>, user_agent=<span class="string">'Scrapy'</span>)</span></span><span class="symbol">:</span></span><br><span class="line">        <span class="keyword">self</span>.user_agent = user_agent</span><br><span class="line"></span><br><span class="line">    @classmethod</span><br><span class="line">    <span class="function"><span class="keyword">def</span> <span class="title">from_crawler</span><span class="params">(cls, crawler)</span></span><span class="symbol">:</span></span><br><span class="line">        o = cls(crawler.settings[<span class="string">'USER_AGENT'</span>])</span><br><span class="line">        crawler.signals.connect(o.spider_opened, signal=signals.spider_opened)</span><br><span class="line">        <span class="keyword">return</span> o</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">def</span> <span class="title">spider_opened</span><span class="params">(<span class="keyword">self</span>, spider)</span></span><span class="symbol">:</span></span><br><span class="line">        <span class="keyword">self</span>.user_agent = getattr(spider, <span class="string">'user_agent'</span>, <span class="keyword">self</span>.user_agent)</span><br><span class="line"></span><br><span class="line">    <span class="function"><span class="keyword">def</span> <span class="title">process_request</span><span class="params">(<span class="keyword">self</span>, request, spider)</span></span><span class="symbol">:</span></span><br><span class="line">        <span class="keyword">if</span> <span class="keyword">self</span>.<span class="symbol">user_agent:</span></span><br><span class="line">            request.headers.setdefault(b<span class="string">'User-Agent'</span>, <span class="keyword">self</span>.user_agent)</span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>在 from_crawler() 方法中，首先尝试获取 settings 里面 USER_AGENT，然后把 USER_AGENT 传递给<strong>init</strong>() 方法进行初始化，其参数就是 user_agent。如果没有传递 USER_AGENT 参数就默认设置为 Scrapy 字符串。我们新建的项目没有设置 USER_AGENT，所以这里的 user_agent 变量就是 Scrapy。接下来，在 process_request() 方法中，将 user-agent 变量设置为 headers 变量的一个属性，这样就成功设置了 User-Agent。因此，User-Agent 就是通过此 Downloader Middleware 的 process_request() 方法设置的。 修改请求时的 User-Agent 可以有两种方式：一是修改 settings 里面的 USER_AGENT 变量；二是通过 Downloader Middleware 的 process_request() 方法来修改。 第一种方法非常简单，我们只需要在 setting.py 里面加一行 USER_AGENT 的定义即可：</p>
                <figure class="highlight lsl">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line">USER_AGENT = 'Mozilla/<span class="number">5.0</span> (Macintosh; Intel Mac OS X <span class="number">10</span>_12_6) AppleWebKit/<span class="number">537.36</span> (KHTML, like Gecko) Chrome/<span class="number">59.0</span><span class="number">.3071</span><span class="number">.115</span> Safari/<span class="number">537.36</span>'</span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>一般推荐使用此方法来设置。但是如果想设置得更灵活，比如设置随机的 User-Agent，那就需要借助 Downloader Middleware 了。所以接下来我们用 Downloader Middleware 实现一个随机 User-Agent 的设置。 在 middlewares.py 里面添加一个 RandomUserAgentMiddleware 的类，如下所示：</p>
                <figure class="highlight haskell">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line"><span class="keyword">import</span> random</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="type">RandomUserAgentMiddleware</span>():</span></span><br><span class="line"><span class="class">    def __init__(<span class="title">self</span>):</span></span><br><span class="line"><span class="class">        self.user_agents = ['<span class="type">Mozilla</span>/5.0 (<span class="type">Windows</span>; <span class="type">U</span>; <span class="type">MSIE</span> 9.0; <span class="type">Windows</span> <span class="type">NT</span> 9.0; <span class="title">en</span>-<span class="type">US</span>)',</span></span><br><span class="line"><span class="class">            '<span class="type">Mozilla</span>/5.0 (<span class="type">Windows</span> <span class="type">NT</span> 6.1) <span class="type">AppleWebKit</span>/537.2 (<span class="type">KHTML</span>, <span class="title">like</span> <span class="type">Gecko</span>) <span class="type">Chrome</span>/22.0.1216.0 <span class="type">Safari</span>/537.2',</span></span><br><span class="line"><span class="class">            '<span class="type">Mozilla</span>/5.0 (<span class="type">X11</span>; <span class="type">Ubuntu</span>; <span class="type">Linux</span> <span class="title">i686</span>; <span class="title">rv</span>:15.0) <span class="type">Gecko</span>/20100101 <span class="type">Firefox</span>/15.0.1'</span></span><br><span class="line"><span class="class">        ]</span></span><br><span class="line"><span class="class"></span></span><br><span class="line"><span class="class">    def process_request(<span class="title">self</span>, <span class="title">request</span>, <span class="title">spider</span>):</span></span><br><span class="line"><span class="class">        request.headers['<span class="type">User</span>-<span class="type">Agent'</span>] = random.choice(<span class="title">self</span>.<span class="title">user_agents</span>)</span></span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>我们首先在类的 <strong>init</strong>() 方法中定义了三个不同的 User-Agent，并用一个列表来表示。接下来实现了 process_request() 方法，它有一个参数 request，我们直接修改 request 的属性即可。在这里我们直接设置了 request 对象的 headers 属性的 User-Agent，设置内容是随机选择的 User-Agent，这样一个 Downloader Middleware 就写好了。 不过，要使之生效我们还需要再去调用这个 Downloader Middleware。在 settings.py 中，将 DOWNLOADER_MIDDLEWARES 取消注释，并设置成如下内容：</p>
                <figure class="highlight ini">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line"><span class="attr">DOWNLOADER_MIDDLEWARES</span> = &#123;<span class="string">'scrapydownloadertest.middlewares.RandomUserAgentMiddleware'</span>: <span class="number">543</span>,&#125;</span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>接下来我们重新运行 Spider，就可以看到 User-Agent 被成功修改为列表中所定义的随机的一个 User-Agent 了：</p>
                <figure class="highlight json">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line">&#123;<span class="attr">"args"</span>: &#123;&#125;, </span><br><span class="line">  <span class="attr">"headers"</span>: &#123;</span><br><span class="line">    <span class="attr">"Accept"</span>: <span class="string">"text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8"</span>, </span><br><span class="line">    <span class="attr">"Accept-Encoding"</span>: <span class="string">"gzip,deflate,br"</span>, </span><br><span class="line">    <span class="attr">"Accept-Language"</span>: <span class="string">"en"</span>, </span><br><span class="line">    <span class="attr">"Connection"</span>: <span class="string">"close"</span>, </span><br><span class="line">    <span class="attr">"Host"</span>: <span class="string">"httpbin.org"</span>, </span><br><span class="line">    <span class="attr">"User-Agent"</span>: <span class="string">"Mozilla/5.0 (Windows; U; MSIE 9.0; Windows NT 9.0; en-US)"</span></span><br><span class="line">  &#125;, </span><br><span class="line">  <span class="attr">"origin"</span>: <span class="string">"60.207.237.85"</span>, </span><br><span class="line">  <span class="attr">"url"</span>: <span class="string">"http://httpbin.org/get"</span></span><br><span class="line">&#125;</span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>我们就通过实现 Downloader Middleware 并利用 process_request() 方法成功设置了随机的 User-Agent。 另外，Downloader Middleware 还有 process_response() 方法。Downloader 对 Request 执行下载之后会得到 Response，随后 Scrapy 引擎会将 Response 发送回 Spider 进行处理。但是在 Response 被发送给 Spider 之前，我们同样可以使用 process_response() 方法对 Response 进行处理。比如这里修改一下 Response 的状态码，在 RandomUserAgentMiddleware 添加如下代码：</p>
                <figure class="highlight ruby">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line"><span class="function"><span class="keyword">def</span> <span class="title">process_response</span><span class="params">(<span class="keyword">self</span>, request, response, spider)</span></span><span class="symbol">:</span></span><br><span class="line">    response.status = <span class="number">201</span></span><br><span class="line">    <span class="keyword">return</span> response</span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>我们将 response 对象的 status 属性修改为 201，随后将 response 返回，这个被修改后的 Response 就会被发送到 Spider。 我们再在 Spider 里面输出修改后的状态码，在 parse() 方法中添加如下的输出语句：</p>
                <figure class="highlight gauss">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line">self.logger.<span class="keyword">debug</span>('Status <span class="built_in">Code</span>: ' + <span class="built_in">str</span>(response.status))</span><br></pre>
                      </td>
                    </tr>
                  </table>
                </figure>
                <p>重新运行之后，控制台输出了如下内容：</p>
                <figure class="highlight gauss">
                  <table>
                    <tr>
                      <td class="gutter">
                        <pre><span class="line">1</span><br></pre>
                      </td>
                      <td class="code">
                        <pre><span class="line">[httpbin] <span class="keyword">DEBUG</span>: Status <span class="built_in">Code</span>: <span class="number">201</span></span><br></pre>
                      </td>
                    </tr>
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                <p>可以发现，Response 的状态码成功修改了。 因此要想对 Response 进行后处理，就可以借助于 process_response() 方法。 另外还有一个 process_exception() 方法，它是用来处理异常的方法。如果需要异常处理的话，我们可以调用此方法。不过这个方法的使用频率相对低一些，在此不用实例演示。</p>
                <h3 id="4-本节代码"><a href="#4-本节代码" class="headerlink" title="4. 本节代码"></a>4. 本节代码</h3>
                <p>本节源代码为：<a href="https://github.com/Python3WebSpider/ScrapyDownloaderTest" target="_blank" rel="noopener">https://github.com/Python3WebSpider/ScrapyDownloaderTest</a>。</p>
                <h3 id="5-结语"><a href="#5-结语" class="headerlink" title="5. 结语"></a>5. 结语</h3>
                <p>本节讲解了 Downloader Middleware 的基本用法。此组件非常重要，是做异常处理和应对反爬处理的核心。后面我们会在实战中应用此组件来处理代理、Cookies 等内容。</p>
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